Concurrent Systems II - Bad Request - Trinity College Dublin

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Scheduling algorithms • Shortest-Job-First [preemptive or non-preemptive] ◾ To see how the value for τn+1 is calculated based on past history, we can expand the formula, substituting for τn τn+1 = αtn + (1- α) αtn-1 + … + (1- α) j αtn-j + … + (1- α) n+1 τ0 α is typically given the value 0.5 ◾ SJF can be either preemptive or non-preemptive • Preemptive SJF will preempt the currently executing process if a process joins the ready queue with a smaller next CPU burst time • Non-preemptive SJF scheduling will allow the currently executing process to complete its CPU burst before scheduling the next process 200 Trinity College Dublin © Mike Brady 2007–2009
Scheduling algorithms • Priority scheduling [preemptive or non-preemptive] ◾ Shortest-Job-First is a special case of a more general priority-based scheduling scheme ◾ Each process has an associated priority ◾ The process with the highest priority is executed on the CPU ◾ Processes with the same priority are executed in the order in which they appear on the ready queue (FCFS order) ◾ What do we mean by high and low priority • We’ll assume high-priority processes have lower priority values than lower priority processes • Doesn’t matter ◾ The priority of a process can be defined either internally or externally • Internally defined priories are based on measurements associated with the process, such as the ratio of I/O burst times to CPU burst times, the estimated next CPU burst time, time since last scheduling • Externally defined priorities are based on properties external to the operating system: user status, wealth, political affiliation, … 201 Trinity College Dublin © Mike Brady 2007–2009
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Concurrent Systems II (CS3D4/CS3BA2
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POSIX Threads • POSIX Threads aka
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Six Separate Threads Master Thread
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Where… • ‘thread’ is the ID
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Hello World -- Creating Threads int
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Hello World -- Thread Function void
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Wilde/Stoker/Ubuntu h-3.1$ cc -lpth
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Interaction • No thread interacti
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Interactions & Dependencies • Int
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…to do Parallel Programming • H
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Hardware Main Memory Level 2 Cache
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Shared Memory Machine • Each proc
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Concurrent Program • A Concurrent
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Communication • A parallel progra
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1 - Flow Dependence S1: write to lo
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3 - Output Dependence S1: write to
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Dependence in Loops j k k i 1 2 j 5
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How can we transform it? • We can
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Issues: • The variable sum, as wr
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Summarising: • We could paralleli
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Example • Sample function: y = 0.
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Synchronisation • We’ve looked
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Shared Variable S: Thread 1 after T
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Mutual Exclusion in pthreads. • M
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Shared Variable S Protected by Mute
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Lock and Unlock Mutex • pthread_m
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} /********** Critical Section ****
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Problem Tutorial • The transpose
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Producers and Consumers • Essenti
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Condition Variables (2) • Dramati
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From Thread U’s POV (2) • If C
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From Thread U’s POV (4) • Acqui
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Thread S (2) • Acquire Mutex M
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Semaphores • Computer Semaphores
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Semaphore Consumer ◾Acquire Mutex
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Semaphores (3) • Useful for count
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Sample Solution for ∏ • Calcula
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unsigned long long clock_cycles() {
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The Challenge of Concurrency • Co
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Concurrent Execution • A concurre
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Notation Trivial Concurrent Program
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Trivial Concurrent Program p1: n
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Example — Jumping Frogs • A fro
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If the frogs can only move “forwa
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State Diagrams for Processes • A
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Book Reference Principles of concur
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Correctness • (Correctness in Seq
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Using State machines • We can des
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The Critical Section Problem (2)
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Observations & Implications • Rem
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Simplify Critical Section Problem i
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State Transitions r1,s1,1 r1: await
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Deadlock Free? • Deadlock Free: I
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Scorecard Mutual Exclusion Deadlock
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Attempt #2 Critical Section Problem
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Attempt #2 Fails… • Attempt 2 f
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Attempt #3 Critical Section Problem
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Attempt #4 loop forever Critical Se
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Attempt #5, derived from #4 Critica
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Verification of concurrent programs
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A quick overview of some mathematic
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A quick overview of some mathematic
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Proving correctness by induction
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A quick overview of some temporal l
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A quick overview of some temporal l
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Temporal logic at work • Consider
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Promela • Promela [Protocol/Proce
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Process (2) • A process type (pro
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SPIN Hello World Example SPIN = Sim
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Promela Variables (2) • Arrays
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Statements (2) • The skip stateme
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If Statement if :: (n % 2 != 0) ->
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Page 151 and 152: Process Execution / Evaluation Sema
Page 153 and 154: The Closed World Assumption • The
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Page 163 and 164: E.g. Virtual memory implementation
Page 165 and 166: Multi-level page tables Virtual add
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Page 169 and 170: Translation look aside buffer • E
Page 171 and 172: Page fault handling Start Access pa
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Page 175 and 176: Page replacement policy • A page
Page 177 and 178: Page replacement policy • First-I
Page 179 and 180: Clock Algorithm 179 Trinity College
Page 181 and 182: Resident set management • Variabl
Page 183 and 184: Resident set management Local Repla
Page 185 and 186: Resident set management Δ Working
Page 187 and 188: Resident set management 187 Trinity
Page 189 and 190: Cleaning policy • Page buffering
Page 191 and 192: Load control • We can use a few d
Page 193 and 194: Process state transitions • Proce
Page 195 and 196: Preemptive and nonpreemptive schedu
Page 197 and 198: Scheduling algorithms • First-Com
Page 199: Scheduling algorithms • Shortest-
Page 203 and 204: Scheduling algorithms • Round-Rob
Page 205 and 206: Example Process Arrival Time Next B
Page 207 and 208: Scheduling algorithms • Multi-lev
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Page 211 and 212: Multi-processor scheduling • Flyn
Page 213 and 214: Layered file I/O (1) Application Di
Page 215 and 216: Disk access • Disks are slow ◾
Page 217 and 218: Disk scheduling • So, the order i
Page 219 and 220: Disk scheduling algorithms (2) •
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